Coal is a commonly used fuel in power generating systems. It is also commonly used in pulverized fuel facilities, and in Integrated Gas Turbine Combined Cycle (IGCC) facilities. However, coal is one of the most impure fuels having impurities that range from trace quantities of many metals, including mercury, small quantities of uranium and thorium, to much larger quantities of aluminum and iron, to still larger quantities of impurities such as sulfur.
Mercury is a toxic material and is released into the atmosphere when coal, or any combustible material (such as a hazardous waste) that may contain mercury, is burned. It has been estimated that approximately 50 tons of mercury are introduced into the environment every year in the US and several thousand tons annually worldwide, a significant portion of it originating from coal fired power plants. Accordingly, there is substantial pressure to reduce the emissions of mercury from coal burning plants.
Some of this mercury is believed to be present in the form of HgCl2, some as elemental mercury vapor (Hg0), and some in different oxidation states. The amount of elemental mercury vapor present varies widely. Concentrations can range from 1 ppb to 1 ppm.
The combustion gas can contain a mixture of CO2, O2, NO, NO2, SO2, CO, H2O, and various toxic metals including mercury. The mechanism of mercury capture on a high surface area sorbent may be influenced by the presence of one or more other combustion by-products, such as disclosed in “Mechanisms of Mercury Capture and Breakthrough on Activated Carbon Sorbents” by Edwin S. Olson, Grant E. Dunham, Ramesh K. Sharma, Stanly J. Miller, Energy and Environmental Research Center, University of North Dakota, Grand Forks, N. Dak., 58202, American Chemical Society, 220th National Meeting, Aug. 20-24, 2000, preprint No. 4 Vol. 25.
One technique proposed for controlling mercury release is to inject a high surface-area adsorbent into the exhaust gas path, then to capture the sorbent on a filter bed, such as a baghouse or similar filter media. This process shows some merit, with theoretically high degrees of mercury capture. However, test results have shown inhibition of the process due to the NO2 in the exhaust gas preventing the uptake of mercury vapor by the sorbent media.